James McAvoy has already worked with Rocket Pictures—he was one of the voice leads in Gnomeo—and though he isn't a phenomenal singer, he can carry a tune well enough to pull off a young Elton.
Perhaps the most fruitful collaboration of Elton's career—besides with Furnish, of course—is his partnership with lyricist Bernie Taupin, who's been Elton's songwriting partner on more than 30 albums. Edward Furlong (American History X, The Green Hornet) has a ton of work lined up for the next year or two—11 projects!—but we think this supporting star could play Taupin with agreeable ease.
We wouldn't be surprised if the film needs at least two Eltons—the man's career spans almost five decades, after all—so for a slightly older version, we concur with Deadline that Ewan McGregor is a natural fit. He looks strikingly like mid-career Elton—all he needs is a little gap tooth—and Moulin Rouge proved he's handy with an Elton tune.
Starting with The Lion King in 1994, through Aida in 1999 and The Road to El Dorado in 2000, Elton's collaboration with Tim Rice has been a surprising and lucrative one. Rice's wry sense of humor would be well-served by John Lithgow.
Elton's friendship with Princess Diana of Wales ended in tragedy; his tribute to her, "Candle in the Wind," was one of the most touching moments during her 1997 funeral. Carey Mulligan could provide the kind of reserved, quiet grace the role demands.
Elton and John Lennon became chummy in 1974; he covered the Beatles' "Lucy in the Sky with Diamonds" and was featured on "Whatever Gets You thru the Night." Throw a pair of little round sunglasses on Edward Norton, and we think he'd make a respectable Lennon.